A multi-producer, single-consumer, futures-aware, FIFO queue with back pressure.

A channel can be used as a communication primitive between tasks running on
futures-rs executors. Channel creation provides Receiver and Sender
handles. Receiver implements Stream and allows a task to read values
out of the channel. If there is no message to read from the channel, the
current task will be notified when a new value is sent. Sender implements
the Sink trait and allows a task to send messages into the channel. If
the channel is at capacity, then send will be rejected and the task will be
notified when additional capacity is available.

When all Sender handles have been dropped, it is no longer possible to
send values into the channel. This is considered the termination event of
the stream. As such, Sender::poll will return Ok(Ready(None)).

If the receiver handle is dropped, then messages can no longer be read out
of the channel. In this case, a send will result in an error.

If the Receiver is simply dropped, then it is possible for there to be
messages still in the channel that will not be processed. As such, it is
usually desirable to perform a "clean" shutdown. To do this, the receiver
will first call close, which will prevent any further messages to be sent
into the channel. Then, the receiver consumes the channel to completion, at
which point the receiver can be dropped.